The 4-way valve used to control the operation of a pneumatic motor can be either mechanically operated or pneumatically operated. When the valve is pneumatically operated, the valve is actuated by a pilot system; usually a pair of pilot valves that sense when the pump is at one extreme or the other of the pump's travel, see FIG. 1.
Current methods to pneumatically detent the pilot valves and create the desired hysteresis involve incorporating a storage springs and a poppet valve, see FIG. 2. When the valve is at rest, the air pressure above the poppet valve is higher then the pressure bellow it, which is usually atmospheric pressure. This differential pressure creates a net force that keeps the valve closed. To actuate the valve, the actuation pin is pressed, which compresses the storage spring until the pin comes in contact with the poppet valve. At this point the valve starts to open and the pressure above the poppet valve should start to drop. When this pressure drops, the force that keeps the valve closed is no longer present and the storage spring can expand back out to its nominal length and snap the valve open, creating the desired hysteresis.
Problems can occur with this method when the pressure above the poppet valve does not drop due to such variables as system volume, leaks, etc. If the pressure does not drop, the pilot valve does not snap open and create the desired hysteresis.